Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Feb 8, 2023 at 2:01 PM John Stultz <jstultz@google.com> wrote:
>
> On Sat, Nov 20, 2021 at 11:27:38AM +0000, Yafang Shao wrote:
> > As the sched:sched_switch tracepoint args are derived from the kernel,
> > we'd better make it same with the kernel. So the macro TASK_COMM_LEN is
> > converted to type enum, then all the BPF programs can get it through BTF.
> >
> > The BPF program which wants to use TASK_COMM_LEN should include the header
> > vmlinux.h. Regarding the test_stacktrace_map and test_tracepoint, as the
> > type defined in linux/bpf.h are also defined in vmlinux.h, so we don't
> > need to include linux/bpf.h again.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Acked-by: Andrii Nakryiko <andrii@kernel.org>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > Cc: Michal Miroslaw <mirq-linux@rere.qmqm.pl>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Petr Mladek <pmladek@suse.com>
> > ---
> >  include/linux/sched.h                                   | 9 +++++++--
> >  tools/testing/selftests/bpf/progs/test_stacktrace_map.c | 6 +++---
> >  tools/testing/selftests/bpf/progs/test_tracepoint.c     | 6 +++---
> >  3 files changed, 13 insertions(+), 8 deletions(-)
>
> Hey all,
>   I know this is a little late, but I recently got a report that
> this change was causiing older versions of perfetto to stop
> working.
>
> Apparently newer versions of perfetto has worked around this
> via the following changes:
>   https://android.googlesource.com/platform/external/perfetto/+/c717c93131b1b6e3705a11092a70ac47c78b731d%5E%21/
>   https://android.googlesource.com/platform/external/perfetto/+/160a504ad5c91a227e55f84d3e5d3fe22af7c2bb%5E%21/
>
> But for older versions of perfetto, reverting upstream commit
> 3087c61ed2c4 ("tools/testing/selftests/bpf: replace open-coded 16
> with TASK_COMM_LEN") is necessary to get it back to working.
>
> I haven't dug very far into the details, and obviously this doesn't
> break with the updated perfetto, but from a high level this does
> seem to be a breaking-userland regression.
>
> So I wanted to reach out to see if there was more context for this
> breakage? I don't want to raise a unnecessary stink if this was
> an unfortuante but forced situation.

Let me understand what you're saying...

The commit 3087c61ed2c4 did

-/* Task command name length: */
-#define TASK_COMM_LEN                  16
+/*
+ * Define the task command name length as enum, then it can be visible to
+ * BPF programs.
+ */
+enum {
+       TASK_COMM_LEN = 16,
+};


and that caused:

cat /sys/kernel/debug/tracing/events/task/task_newtask/format

to print
field:char comm[TASK_COMM_LEN];    offset:12;    size:16;    signed:0;
instead of
field:char comm[16];    offset:12;    size:16;    signed:0;

so the ftrace parsing android tracing tool had to do:

-  if (Match(type_and_name.c_str(), R"(char [a-zA-Z_]+\[[0-9]+\])")) {
+  if (Match(type_and_name.c_str(),
+            R"(char [a-zA-Z_][a-zA-Z_0-9]*\[[a-zA-Z_0-9]+\])")) {

to workaround this change.
Right?

And what are you proposing?